A dust core produced via pressure-forming of a powder for a dust core comprising a soft magnetic metal powder is applied to a stator core or a rotor core of a vehicle driving motor, a reactor core that constitutes a power inverter circuit, and the like. Unlike a core member composed via lamination of electromagnetic steel sheets, the dust core has many advantages such that: it has magnetic properties such as low high-frequency iron loss; it can be formed into a variety of shapes in a flexible manner at low cost; and the material cost is lower than those of alternatives.
Regarding the above-mentioned dust core, a measure exists to increase the specific resistance for reduction of iron loss, and particularly eddy loss, involving the preparation of an iron alloy of iron and silicon, aluminium, or the like as a soft magnetic metal powder, the formation of an insulating film of silica (SiO2) or the like on the surface layer so as to prepare a magnetic powder, and the subsequent production of a dust core by pressure forming the magnetic powder. However, preparation of a magnetic powder using an iron alloy in which silicon, aluminium, or the like is homogeneously dispersed in an iron powder results in a problem such that the resulting hardness is excessively high and the realization of a high density for the dust core (produced via pressure forming thereof) is actually inhibited. If the density of the dust core cannot be increased, the magnetic flux density of the dust core cannot be increased. Therefore, it has been conventionally difficult to produce a dust core with high density, high specific resistance, and high magnetic flux density. A method that has been desired comprises infiltrating the surface layer of a soft magnetic metal powder with a silicon element or the like in an amount that results in as thin a state as is possible so as to enhance the specific resistance, and thus preparing a powder for a dust core in which no or an extremely small amount of a silicon element or the like is present.
For example, Patent document 1 discloses a method for producing a silicon layer-coated iron powder with a surface layer having a high concentration of silicon, which comprises mixing an iron powder subjected in advance to high temperature treatment and pulverization with a silicon powder and ferrosilicon and then performing high temperature treatment again in a hydrogen atmosphere.    Patent Document 1: JP Patent Publication (Kokai) No. 2007-126696 A